One attractive class of modern high energy density rechargeable cells is the Lithium-ion (Li-ion) cell. The principle components of a Li-ion cell are graphitic carbon anode, for example, natural or artificial graphite, a typical example being mesocarbon microbead (MCMB) carbon, a lithiated transition metal oxide cathode such as LiCoO2, and a highly conductive electrolyte solution. The electrolyte provides mobility to the Li ions, which are transported from the anode to the cathode, and vice versa, during discharge and charge of the battery. The electrolyte in a Li-ion cell is composed of a lithium salt that is dissolved in a nonaqueous solvent such as an organic carbonate(s). To a large extent, the salt used in the electrolyte of the cell governs the overall performance of the cell and the salt must therefore meet certain requirements. In terms of performance, a salt must have high conductivity, high thermal stability, and electrochemical stability above the potential of the fully charged cell (4.1 V vs. Li in cells employing carbon anode materials), and be nontoxic and safe.
Unfortunately, no salts adequately meet all the cost, performance, and safety requirements imposed by the industry. The most common salt in use today is LiPF6, which is added to organic carbonate solvent mixtures to form the electrolyte solution. This salt has excellent conductivity and electrochemical stability in these solvents but is expensive. In addition, this salt is limited to an operational temperature range of −40° C. to +50° C. The LiPF6 is thermally unstable and is believed to decompose at temperatures above 60° C. according Equation 1 below. In addition, both LiPF6 and PF5 are susceptible to hydrolysis and, as a result, they will react with any moisture in the electrolyte according to Equations 2 and 3 to form HF.LiPF6+H2O→POF3+2HF+LiF  (Equation 2) PF5+H2O→POF3+2HF  (Equation 3) 
The HF and PF5 can catalyze the decomposition of the solvents, react with the electrodes to increase the electrode/electrolyte interfacial impedance, and corrode the current collectors. Other lithium salts based on perfluorinated inorganic anions with the general formula LiMFx, have been extensively studied. The order of conductivity of these salts is LiSbF6>LiAsF6≈LiPF6>LiBF4. However, each of these salts has either poor electrochemical stability (LiSbF6), toxicity (LiAsF6), or poor cycling efficiency (LiBF4).
The recent development of several organic anions, some of which have high conductivities, has overcome some of the performance problems with the inorganic anions. The most promising group of these anions is that based on fluorinated sulfonyl ligands. The Li salt of N(SO2CF3)2−, for example, is highly conductive and thermally stable to 360° C. However, it has been reported to corrode aluminum at high potentials which is a problem for cells employing aluminum current collectors. Other related salts being investigated include LiC(SO2CF3)3 and those obtained by the substitution of various fluorinated organic groups (R) on LiN(SO2R)2. While these anions have promising performance characteristics, they are expensive and the need for an inexpensive salt remains unsatisfied.
U.S. Pat. No. 6,022,643 issued to Hung S. Lee et al. on Feb. 8, 2000, assigned to Brookhaven National Laboratory, discloses that the addition of a three-coordinate boron compound to a lithium salt in organic carbonate solutions dramatically increases the conductivity of the lithium salt. The lithium salts, LiF, CF3CO2Li, and C2F5CO2Li, were combined with various organofluorine boron based compounds. The patentees referred to the three coordinate boron based compounds as “anion receptors” because they would seek a fourth ligand from the salt anion, thus increasing the conductance and Li transference number. While these solutions are conductive and electrochemically stable over the necessary potential range, they require the use of an expensive Lewis acid in a 1:1 ratio with the lithium salt, which increases the cost of the electrolyte.
U.S. Pat. No. 6,395,671 issued to Robert E. LaPointe, assigned to The Dow Chemical Company, discloses that the addition of two Lewis acids to a monoanionic species with two Lewis basic sites yields an anion that is only very weakly Lewis basic. Potassium and ammonium salts of these anions were prepared, and the ammonium salts were used in the preparation of olefin polymerization catalysts, which requires that the anion be dissociated from cation. The dissociation of the anion from the cation (ie. low degree of ion-pairing) is also important in achieving a highly conductive lithium salt. However, the synthetic routes to the salts shown below in Equations 4 and 5 do not include a synthetic route to a lithium salt. 